1. Field
The present invention relates generally to wireless power, and more specifically, to selective transfer of wireless power.
2. Background
Typically, each battery powered device requires its own charger and power source, which is usually an AC power outlet. This becomes unwieldy when many devices need charging.
Approaches are being developed that use over the air power transmission between a transmitter and the device to be charged. These generally fall into two categories. One is based on the coupling of plane wave radiation (also called far-field radiation) between a transmit antenna and receive antenna on the device to be charged which collects the radiated power and rectifies it for charging the battery. Antennas are generally of resonant length in order to improve the coupling efficiency. This approach suffers from the fact that the power coupling falls off quickly with distance between the antennas. So charging over reasonable distances (e.g., >1-2 m) becomes difficult. Additionally, since the system radiates plane waves, unintentional radiation can interfere with other systems if not properly controlled through filtering.
Other approaches are based on inductive coupling between a transmit antenna embedded, for example, in a “charging” mat or surface and a receive antenna plus rectifying circuit embedded in the host device to be charged. This approach has the disadvantage that the spacing between transmit and receive antennas must be very close (e.g. mms). Though this approach does have the capability to simultaneously charge multiple devices in the same area, this area is typically small, hence the user must locate the devices to a specific area.
As will be appreciated by a person having ordinary skill in the art, wireless charging may be achieved via an unmodulated signal, which allows narrow tuning of a transmitter and a receiver to the same frequency. However, transmission of such narrowband signal may allow any receiver, including unauthorized receivers, in the vicinity of the transmitter, to tune to the same frequency, taking charge and disrupting charging or authorized receivers.
A need exists for providing wireless power to authorized users while restricting access to unauthorized users.